1. Field of the Invention
This invention relates generally to antenna arrays and particularly to antenna arrays for determining an angle of arrival of an incident electromagnetic wave. Still more particularly, this invention relates to an antenna array that is configured on a perimeter portion of an airframe
2. Description of the Prior Art
Previous interferometric direction finding antennas typically are either externally mounted on an airframe or are center configured. Externally mounted antennas are bulky and introduce aerodynamic drag that diminishes performance. Center configured antennas are difficult to use with other sensors. Center configured antennas can detect only one circular polarization (left or right), which limits their utility in that the opposite sense circular polarization (right or left) cannot be detected.
Suitable surface mounted antennas are linearly polarized, which creates a problem with tracking sources of different polarizations. When the airframe rolls, a linearly polarized antenna becomes cross-polarized relative to the incident wave, which causes the antenna to produce no signal or a noisy low power signal in response to the cross-polarized wave.
The present invention overcomes problems associated with prior art direction finding antenna arrays and solves the cross-polarization problem by using pairs of antennas having different linear polarizations such that no incident polarization can be cross-polarized. The preferred arrangement of an antenna array according to the present invention includes two pairs of linear polarized antennas with one pair having a polarization normal to the airframe and the other pair having a polarization tangential to the airframe.
An object of the invention is to provide a perimeter configured direction finding (DF) antenna array that is capable of providing orthogonal plane direction finding and polarization diversity over a wide bandwidth.
A direction finding antenna array according to the invention comprises a first pair of linearly polarized antennas mounted on opposite sides of a perimeter portion of an airframe such that their polarizations are tangentially directed and are perpendicular to a longitudinal axis of the airframe. A second pair of linearly polarized antennas is mounted to the airframe such their polarizations are normally directed and such that each of the second pair of antennas is equidistant from the first pair of antennas.
The first and second antennas may be formed as log periodic folded slot antennas.
The third and fourth antennas may be formed as flared notch antennas. Alternatively, the third and fourth antennas may be formed as log periodic folded dipole antennas or other log periodic type antenna.
A method according to the invention for determining an angle of arrival xcex8 of an electromagnetic wave having a wave polarization that is incident upon an antenna array mounted to a perimeter portion of an airframe comprises the steps of mounting a first pair of linearly polarized antennas on opposite sides of a perimeter portion of an airframe such that their polarizations are tangentially directed and are perpendicular to a longitudinal axis of the airframe, and mounting a second pair of linearly polarized antennas to the airframe such their polarizations are normally directed and such that each of the second pair of antennas is equidistant from the first pair of antennas. The method of the invention further comprises the step of determining the angle of arrival xcex8 measuring phase differences xcex94xcexa8=xcexa82xe2x88x92xcexa81=(2xcfx80/xcexd2 sin xcex8+xcex12)xe2x88x92(2xcfx80/xcexd1 sin xcex8+xcex11), xcex11 and xcex12 being phase angles of normalized complex voltages v1, and v2 given by:
xcexd1=ewxc2x7ea1*=ejxcex11xe2x80x83xe2x80x83(1)
and
xcexd2=ewxc2x7ea2*=ejxcex12xe2x80x83xe2x80x83(2)
with ew and ea representing complex vectors of the wave and antenna polarizations, respectively.
The structure and function of the invention may be best understood by referring to the accompanying drawings, which are not to scale, and to the following detailed description.
For a continuously rotating airframe, the phase difference is:                     ψ        =                                            2              ⁢              π                        λ                    ⁢          d          ⁢                      xe2x80x83                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      θ            source                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      φ            roll                                              (        3        )            
where xcfx86roll is the roll angle. The unambiguous angle of arrival can be determined using the expression:                               θ          source                =                  arcsin          ⁡                      [                                          λ                                  4                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                  d                                            ⁢                              xe2x80x83                            ⁢                              ψ                                  p                  -                  p                                                      ]                                              (        4        )            
xcfx86source is determined by peak or zero locations where xcexa8p-p is the peak to peak value of the unwrapped phase difference in co-polarized regions of the two antenna pairs that have been spliced together effectively replacing the cross-polarized regions. Co-polarized regions are regions throughout a roll that are within 3 dB down from perfectly co-polarized points.
Alternatively, amplitude comparison may be used to solve ambiguities if sufficient squint is obtained. Side by side antennas may be used to solve the ambiguities. Array processing or monpulse techniques may be used.